Map Projections Impact Area, Shape, Distance & Direction
Issue #1:
Issue #2:
Earth to Map
Earth Model
Position of Geoid
Geoid
Ellipsoid
Ellipsoid
Multiple Ellipsoids, Different Mapping Purposes
Point of Tangency
3 Different Development Surfaces
Datum:
A datum is a formal description of the shape of the Earth along with an “anchor” point for the coordinate system.
Geographic coordinate systems consist of a datum, a reference ellipsoid, and an angular unit of measure.
Datum Notation on a Reference Map
Datum Selector in a Typical GPS Unit
What distortion can you give up in favor of the one(s) you really need?
How big is your project area?
What unit of measure (feet, inches, degrees…) would you like your map to be measured in?
What is the end goal of your project?
QGIS CRS Properties
CRS Formats
proj4
Where located for layer ?
Find a CRS in QGIS
A Question Mark Icon means QGIS is unable to determine the Layer CRS
Projects have an underlining CRS; layers can have the same CRS, or different CRS within One Project
Project Properties CRS
QGIS supports “on the fly” CRS transformation for both raster and vector data.
Selecting a CRS, both project and layer:
CRS Selector
Tissot’s indicatrix
WGS84:Preview CRS within QGIS
World_Polyconic:EPSG 54021
Layer Properties > Missing CRS
Warning in QGIS Layers Panel for Missing CRS
Setting a CRS in QGIS
Searching for WGS84 in the QGIS CRS Selector Interface
Reprojecting a Feature within QGIS
Searching for WGS84 in the QGIS CRS Selector Interface
ESPG 4326 WGS 84 project CRS will be used to calculate areal units via Field Calculator. The Field Calculator will use $area to determine areal calculations in square meters. In the current QGIS LTR, the default areal unit for calculations is indeed square meters unless it is changed to a different unit:Default Measurement Units in Current QGIS LTR
Geometry Attributes
ESPG 4326 WGS 84 layer/project crs:Forcing Layer CRS
Resulting Area expressed in Degrees
EPSG 102745, Areal calculations can be determined in square feet as the projection unit is feet:Setting CRS to Vermont’s State Plane Coordinate System
Geometry Attributes based on Layer or Project CRS will result in Area based on Square Feet
Square Meters and Square Feet can be expressed as Square Miles using the following formulas, respectively:
mi² * 0.00000038610
ft² * 0.000000035870
Datum Transformations
Datum Transformations